With
the unit switched OFF, simultaneously press and hold the UP ARROW key
and the ASSIGN key and then turn power on to the unit. The two keys
will have to be pressed until the opening DRAWMER ELECTRONICS message
has cleared (about 4 seconds).

A
prompt will ask RE-CAL?, if the YES key is pressed then the unit will
jump into the second stage of the calibration process. Any key other
than YES will abort the re-calibration.

Software
calibration will require you to connect two balanced cables from the
output to the input of each channel. This appears rather odd to the
normal use of any piece of equipment, but in this case the M500 will
be testing itself and will not cause any damage. When connected up,
hit any key.

The
calibration process commences. After about 30-45 seconds a message
appears which should give a measurement for the four detectors named
A, B, C & D. You should notice the levels passed by the SLAVE
processor to the MAIN processor being displayed in an ascending order.
Eventually a report should be given as to the success of this first
stage of calibration. The detector letters should all have the word OK
against them, or, a low number eg. 1 or 2. The optimum first
calibration check will display:

A
OK C OK CAL DONE

B
OK D OK HIT KEY

Up
to two errors are permissable on any detector. 1 error is equal to 1dB
of un-linearity in a signal level detector somewhere between -50dB and
+20dB, obviously this is fairly insignificant! If any more than 1 or 2
errors is displayed then it is suggested that the unit be powered off
and then back on again to repeat the first half of the re-calibration.
If, for example, the message reads A LOW then the A detector signal
level is low. If A, B, C, and D are LOW then the test tone signal
is probably missing (IC38 Pin 6). Other possible messages are:

If
the calibration results are greatly different from the four OK's,
switch off the M500 unit, check the connecting leads and try again.
The same applies to Filter calibration which follows on from detector
calibration. The optimum filter calibration check will display:

IC3
OK IC4 OK : HIT KEY

IC8
OK IC9 OK :

One
filter error is permissible. (1 error is equal to 3dB un-linearity in
thye filter table). If more than one filter error occurs, then restart
the re-calibration process by switching on and off again. You could
try swapping the LPF with the HPF filter IC blocks on that channel. If
this has to be done take care when removing blocks from the IC bases.